As shown in FIG. 12, since a conventional motor driving device cannot obtain an initial position of a rotor of a motor, the conventional motor driving device has to output a pulse signal randomly to any one of three phases of the motor to move the rotor of the motor from an unknown position to a specified positioning position before driving the motor to run. In a positioning mode as shown in FIG. 12, it can be observed that the positioning of the rotor takes a long time before the motor runs. Also, an excessive startup preparation time does not meet customer demands. Furthermore, in the process of moving the rotor of the motor from the unknown position to the positioning position, the motor would vibrate and the fan would shake, resulting in an excessively high decibel noise. After the motor is positioned, the conventional motor driving device usually drives the motor to run by using a square waveform signal having potentials being significantly changed, and thus generates a louder noise than using a sine waveform.